There exist manifold situations in which a process for reducing the fusion point of coal ash could be gainfully employed. A representative sampling of such situations is listed and discussed below.
1. Daily operation of wet bottom boilers.
In wet bottom boilers such as cyclone and slag tap furnaces, the ash particles resulting from the burning of coal are permitted to collect in the bottom of the furnace box from which the ash is continually removed as a molten liquid. If, for any reason, the molten coal ash or slag does not run, it can very quickly close over the slag drain openings and result in shut-down of the entire furnace. Accordingly, a wet bottom boiler is usually designed with a particular type of coal in mind as the sole fuel for the design, the working assumption being that such coal will be of uniform ash content and that the slag will be of uniform viscosity and fusion point.
Unfortunately, the working assumption is but a working assumption. The ash content of coals varies widely not only in coal from different parts of the world, but even in different seams within the same region, or even in different parts of the same mine. For instance, the bulk of the bituminous coal used for power generation in the United States has an ash content generally within the range of 6-20%, but some such coals have an ash content as high as 30%. Furthermore, the temperature within the furnace box of a slag tap furnace will vary with the operating level of the furnance. For example, during low load operation, even a coal with a slag of medium fusion point may not be suitable for slag tapping since the furnace box temperature may not be sufficiently high to obtain the degree of fluidity necessary for tapping.
Numerous attempts have been made to determine relationships so that one can calculate the slagging tendencies (that is, the ash fusion points and ash viscosities) of a coal ash from its chemical composition. The composition of coal ash is customarily determined by a chemical analysis of the residue which is produced by burning a sample of coal at a slow rate and at a moderate temperature (732.degree. C) under oxidizing conditions in a laboratory furnace. Such analysis reveals that coal ash is composed chiefly of compounds of silicon, aluminum, iron and calcium, with smaller amounts of magnesium, titanium, sodium and potassium. However attempts to calculate parameters such as the fusion point and viscosity of the coal ash from a chemical analysis of the coal ash have left much to be desired, and none of the particular ratios utilized for this purpose (such as the silica ratio, the base-to-acid ratio, etc.) seems to be satisfactory under all conditions.
As knowledge of the factors affecting ash deposition has increased, guidelines have been established to arrive at suitable equipment designs for various fuels. One such guideline is called a "fouling index", which uses a total alkali content in the coal as a criterion. This guideline is primarily useful for predicting fouling in the superheater area resulting from flue gas fly ash, and is unfortunately not of particular value in the prediction or correction of slag tap problems resulting from fusion point or viscosity problems with coal ash. While various studies regarding the correction of such slag tap problems have indicated possible techniques for correction of such slag tap problems, such techniques tend to be effective only with particular ranges of coal composition, create secondary furnace problems of their own, and/or are simply not economically feasible. For example, use of an inexpensive salt such as sodium sulfate as an additive to the coal to be burned presents the danger of hydrogen sulfide generation under certain conditions. The use of soda ash (sodium carbonate) or caustic (sodium hydroxide) is effective only at additive levels which are so high that the amount of sodium introduced presents corrosion problems.
Thus, the need remains not only for a method of lowering the ash fusion point and ash viscosity of coals of known slagging characteristics (so that such coals may be utilized in slag furnaces designed for operation in connection with coals exhibiting better slagging characteristics), but also for a method of modifying such slagging characteristics "on the fly" in response to hour-by-hour variations in the coal composition and operating levels of the slag furnace.
2. Freeing of clogged slag drains in wet bottom boilers.
For a variety of reasons (including fluctuations in the coal composition being fed to the furnace and/or in the operating level of the furnace), high fusion point coal ash may unexpectedly solidify within and close the slag drain openings of a wet bottom boiler. This can require a temporary shutting down of the furnace to permit a re-opening of the slag drain openings. A clearly more acceptable procedure would be to reduce the fusion point of the clogging solidified coal ash so that it again becomes molten and flows out the slag drain openings.
3. Shutdown of wet bottom boilers.
When a wet bottom cyclone furnace is being taken down for a planned or emergency outage or shutdown, the normally molten slag solidifies in the cyclones as the boiler cools. This results in expensive and time consuming cleaning operations to remove the solidified slag from the cyclones before the unit can be restarted. Any means of reducing the amount of slag during the shutdown operation would obviously reduce or even eliminate the hours normally spent on cleaning the cyclones. A method of lowering the ash fusion point would significantly reduce the amount of slag left in the cyclones. Three reasons for this are proposed. First, it would lower the viscosity of the molten slag already present, allowing it to flow more rapidly out of the cyclone; second, while the furnace is still at operating temperatures, any solidified slag present would tend to soften and become fluid; and, third, after the fuel supply has been cut off and the unit starts to cool, the treated slag having a lower fusion point will remain molten and fluid, and thus able to drain for a longer period of time than would be the case for untreated slag.
4. Improved insulation of dry bottom boilers.
During the operation of a furnace some heat is lost by absorption and conductance through the furnace walls. If this loss is excessive, then the exit gas temperature from the furnace falls below the design temperature, steam temperatures drop, and the overall efficiency of the unit decreases. Slag on the furnace walls acts as a thermal insulator and can reduce this heat loss through the furnace walls. In wet bottom coal fired units, molten slag is invariably present during operation and the walls of the furnace are usually at least partially coated with slag. In a dry bottom furnace where, by choice, a coal with a high fusion point ash is burned, the ash is dry and does not tend to stick to and insulate the walls. Use of a coal with a lower ash fusion point is not possible here simply because the furnace is not designed to handle large amounts of molten slag. However, intermittent use of an additive which would lower the fusion point of a small portion of theash and cause localized ash build-up on the furnace walls the ash insulate the furnace walls.
5. Slag removal from walls of wet and dry bottom boilers.
During the normal operation of wet bottom boilers and during the operation of dry bottom boilers as indicated immediately above, excessive accumulations of solidified coal ash can form on the interior walls of a boiler, where they are difficult to remove. These accumulations are frequently referred to as "eyebrows" and can exceed the size of a grand piano. Lowering the fusion point of such eyebrows would allow them to drop off the boiler wall for easy collection and removal.
Accordingly, it is an object of the present invention to provide a method for reducing the fusion point of coal ash in a boiler.
It is another object to provide such a method which can be used to improve the slagging characteristics "on the fly" in response to momentary fluctuation in the coal composition and/or operating level of a wet bottom boiler.
It is a further object to provide such a method which can be used to open slag-clogged drain openings of a wet bottom boiler.
It is also an object to provide such a method which can be used to facilitate or eliminate the need for cleaning of a wet bottom boiler after its shut-down.
Another object is to provide such a process which can be used to improve the effective wall insulation of a dry bottom boiler.
A further object is to provide such a process which can be used to facilitate slag removal from the walls of both wet and dry bottom boilers.
A final object is to provide such a method which is economically feasible and which does not compound other problems associated with boilers.